1. Field of the Invention
The present invention relates generally to channel selection in multi-channel wireless communications networks.
2. Description of the Related Art
Advances in computing and communication technologies have resulted in smaller, faster, economical, and more reliable network devices that enable communication among mobile users. Due to a crowded and harsh electromagnetic environment (e.g., weather, terrain features, jamming, interference and manmade obstructions) networking challenges include high latency, low bandwidth and poor reliability. Prior art multi-channel networks added a degree of freedom to wireless communications networks by dividing the allocated spectrum into multiple channels resulting in an increase in spatial reuse by accommodating simultaneous transmissions. Multi-channel wireless networks also exploited frequency diversity, and capacity was no longer limited to a single channel. However, challenges surfaced with the use of multiple channels.
In single channel networks, medium access protocols only need to decide when the channel is suitable for communication to reduce the likelihood of collisions between nodes in the medium. When multiple channels are available, a source-destination pair must select and be synchronized to a common idle channel before attempting a transmission. The goal of coordinating nodes was to minimize collisions during data transmission to avoid delay and unnecessary energy usage due to retransmissions.
In an N channel wireless network, M channels may be selected at a given time by a source node to transmit information to a destination node where1≦M≦N.  (1)Four well known prior art medium access methods in multi-channel wireless networks are frequency hopping, time division multiple access (TDMA), multiple transceiver, and dedicated control channel approaches.
Random channel selection is the process of selecting channels for communication between nodes arbitrarily, e.g., in a pseudorandom manner, without regard to channel performance. Accordingly, in a random channel selection method there is a high probability of transmission error when one or more channels are disadvantaged. Resultant from lost transmissions is high network latency, increased retransmissions and decreased throughput.
To illustrate these effects, consider that performance of each data channel in a system can be determined by defining the probability of successful transmission of a packet of size P bits in the ith channel in the system asρi=(1−pei)P  (2)where pei is the probability of bit error for that channel and the bit errors in the packet are assumed to be independent. For example, in the case of Quadrature phase shift keying (QPSK) the probability of bit error for the ith channel can be expressed as
                              p                      e            ⁢                                                  ⁢            i                          =                  Q          ⁡                      (                                                            2                  ⁢                                                                          ⁢                                      E                    b                                                                    I                  0                                                      )                                              (        3        )            where Eb is the energy per bit, and Io=No+Jo is the sum of the power spectral densities of the Additive White Gaussian Noise (AWGN) and jamming present in the channel. Each node in a wireless network has a sequence of N probability of successful transmission values corresponding to N data channels. Nodes can select up to M data channels given an M×N selection criterion during a session resulting in K possible combinations of channel selections given by
                    K        =                              (                                                            N                                                                              M                                                      )                    =                                    N              !                                                      M                !                            ⁢                                                (                                      N                    -                    M                                    )                                !                                                                        (        4        )            
The probability of successful transmission for the system using M×N random channel selection is then given by
                              ρ          =                                    1              K                        ⁢                                          (                                                      ρ                    1                                    ⁢                                                                          ⁢                  …                  ⁢                                                                          ⁢                                      ρ                    M                                                  )                            i                                      ,                  0          ≤          ρ          ≤          1                                    (        5        )            where (ρ1 . . . ρm) is a combination of M probability of successful transmission values corresponding to selected channels. Equation (5) defines a metric that can be used to compare the performances of different multi-channel selection methods against each other.To demonstrate the challenge of random channel selection, consider an M×8 wireless network that can have between one and eight disadvantaged channels. Using Equation (5) while varying the number of disadvantaged channels for 1≦M≦4, the inefficiency of random selection can be quantified as illustrated in FIG. 1. The results illustrated in FIG. 1 demonstrate that for random channel selection as the number of disadvantaged channels increases, the probability of successful transmission decreases due to an inability to identify and favor high performing channels. As the number of disadvantaged channels increases, the probability of successful transmission decreases. When M>1, the probability of successful transmission decreases rapidly as M becomes larger.